Water Environment Research • 1–10, 2019 RESEARCH ARTICLE 1 Interdisciplinary Center of Marine and Environmental Research (CIIMAR/ CIMAR), University of Porto, Matosinhos, Portugal 2 Institut des Sciences Analytiques et de Physicochimie Pour l’environnement et les Matériaux, UMR5254, CNRS/ University of Pau & Pays Adour/ E2S UPPA, Pau, France 3 Department Analytical Chemistry, Science & Technology Faculty, University of Basque Country, UPV/EHU, Bilbao, Spain 4 Science Faculty, University of Porto, Porto, Portugal Received 17 July 2019; Revised 24 September 2019; Accepted 7 October 2019 Foundation for Science and Technology (Portugal); European Regional Development Fund (ERDF), Grant/Award Number: PT2020 Additional Supporting Information may be found in the online version of this article. Correspondence to: Teodor Stoichev, Interdisciplinary Center of Marine and Environmental Research (CIIMAR/CIMAR), University of Porto, Terminal de Cruzeiros de Leixões, Matosinhos, Portugal. Email: tstoichevbg@yahoo.com Robert Duran, Institut des sciences analytiques et de physicochimie pour l’environnement et les matériaux, UMR5254, CNRS/University of Pau & Pays Adour/E2S UPPA, Pau, France. Email: robert.duran@univ-pau.fr DOI: 10.1002/wer.1255 © 2019 Water Environment Federation Modeling phaeopigment concentrations in water from a shallow mesotrophic lagoon Teodor Stoichev , 1 Robert Duran , 2 Alberto de Diego , 3 Vitor M. Vasconcelos 1,4 • Abstract Concentrations of phaeopigments (Pha) and chlorophyll a (Chl) were determined in surface waters from a temperate lagoon during six sampling campaigns at high and at low tide. In order to develop models for phaeopigment concentration in water, it was necessary to replace Chl with photosynthetic pigment concentration (P t  = Pha+Chl) as one of the explanatory variables. Under first approximation, food availability and water temperature (T) could be considered as independent variables. The concentra- tions of Pha were then determined following seasonal change of response curves of the consumer community on T. However, multiple regression models with P t , T and, eventually, salinity as explanatory variables were better able to depict Pha. All equa- tions, developed with P t , were also solved using Chl as an input variable. Although part of the performance was lost, such back-transformed models can be used at low/ medium T and moderate to high concentrations of Chl. The developed equations about middle to long-term variations of Pha could be applied to study the biogeo- chemistry of contaminants related to Pha and to evaluate the dependence on tem- perature of phytoplankton utilization by consumers. © 2019 Water Environment Federation • Practitioner points • Phaeopigment concentration depicted by chlorophyll (Chl), temperature (T), and salinity. • Better results obtained at low to medium T and moderate to high Chl concentration. • Multiple regression (MR) better for extrapolation than model (S) with variable separation. • Thermal response of consumer community in mesotrophic lagoon studied using model S. • Key words coastal lagoons; modeling; phaeopigments; phytoplankton; zooplankton Introduction The level of chlorophyll a (Chl) in water is a widely used indicator for phyto- plankton biomass. The concentration in water of the decomposition products of Chl—the phaeopigments (Pha), is related to a variety of aquatic biogeochemical processes, for example, phytoplankton decay. Consequently, it may reflect higher dissolved organic matter concentrations in estuaries (Luengen & Flegal, 2009; Viličić, Legović, & Žutić, 1989) and seas (Maciejewska & Pempkowiak, 2015). Both Chl and Pha were used as predictors of estuarine copepod density, diversity, and dynamics (Islam, Ueda, & Tanaka, 2006). Pattern of Pha in water is indicative of herbivorous zooplankton grazing in a variety of coastal environments: a shallow temperate estuary (Biancalana, Menéndez, Berasategui, Fernández-Severini, & Hoffmeyer, 2012), a lagoon (Collos et al., 2005), and gulfs (D’Agostino et al., 2018). The heterotrophic bacterial growth could be supported, not only by phytoplankton exudates, but also by the activity of grazers (Pagano et al., 2006; Peduzzi & Herndl, 1992). Such a microbial loop is very important in coastal environments and could be involved in active heterotrophic degradation of phytoplankton, as demonstrated